The changes we experience as we accept Christ are so radical we are considered "new." Second Corinthians 5:17 says, "Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come." This "new creation" is the Greek
kainos
, which means something just made and which is uncommon or novel or unheard of. We are not cleaned out or covered over; we are new. This newness manifests itself in our spiritual condition, our relationships, and our hearts.
Spiritually, in Christ we are alive for the first time. Ephesians 2:1 describes the old self as "dead in [its] trespasses and sins." Our new self is at peace with God (Romans 5:1) and alive in Christ (Ephesians 2:4-5). In addition, God has conformed us to His image (Romans 8:29), and releases us from condemnation (Romans 8:1).
In our relationship with God, we are no longer His enemies, we are His children. Romans 3:10-11 describes the old self: "None is righteous, no, not one; no one understands; no one seeks for God." Romans 5:10 describes the old self as enemies of God. Hosea 2:23 (NIV), however, predicts: "I will say to those called 'Not my people,' 'You are my people'; and they will say, 'You are my God.'" And Galatians 3:26 announces the arrival of our new position in Christ: "for in Christ Jesus you are all sons of God, through faith."
With regard to other people, in Christ we are part of a family. Where once we were "strangers and aliens," we are now "fellow citizens with the saints and members of the household of God" (Ephesians 2:19). The old self is alone, but the new is part of "one body, and each member belongs to all the others" (Romans 12:5 NIV). Not only a part, but an irreplaceable part (1 Corinthians 12:18) of a cohesive whole (1 Corinthians 12:25-26).
Personally, our new hearts mean our character can change. We become less concerned with the world, and more concerned with living as God intends (Romans 12:2). We care more about others, and less about ourselves (Philippians 2:3-4). When we accepted Christ, God automatically took us as His children and established us into the church family. Our heart changes, however, are gradual (2 Corinthians 3:18). We possess a new heart (Ezekiel 11:19), but we need to intentionally train it (2 Peter 1:5-2). This is only possible because the new self is released from sin and death and free to follow God (Romans 8:6-8).
In Christ we are a new creation, and this newness affects every area of our lives. For the first time, we become spiritually alive. We go from being God's enemies to His children. We have a new life as a part of a new family. And we are free to follow God and care about others. In Christ, we become more like Christ.